The present invention relates to a rare earth thin-film magnet of a Nd—Fe—B film formed on a silicon substrate and a method of producing a rare earth thin-film magnet of a Nd—Fe—B film formed via pulsed laser deposition (PLD).
In recent years, pursuant to the weight-saving and downsizing of electronic devices, the miniaturization and sophistication of rare earth magnets having superior magnetic properties are being advanced. Since a neodymium-iron-boron (Nd—Fe—B)-based magnet exhibits the highest maximum energy product among existing magnets, the practical application thereof to MEMS (Micro Electra Mechanical Systems), energy sectors such as energy harvest, and medical device sectors is expected.
This kind of rare earth magnet thin film is known to be produced via the sputtering method (Patent Document 1, Non-Patent Document 1) or via pulsed laser deposition method (Patent Document 2, Non-Patent Document 2) or other PVD (Physical Vapor Deposition) methods (Non-Patent Document 3). In these documents, a rare earth magnet thin film is formed on a metal substrate made from Ta, Mo or the like.
Meanwhile, in order to effectively leverage the lithographic technique based on silicon (Si) semiconductors, the stable formation of a Nd—Fe—B film on a versatile Si substrate is strongly desired upon preparing micro actuators of micro magnetic devices for MEMS.
Non-Patent Document 4 describes that, when a magnetic film having a composition that is equivalent to Nd2Fe14B, which is a stoichiometric composition, is directly deposited on a Si substrate, stress is generated during the heat treatment of the deposited film due to the difference in thermal expansion rate between the Si substrate and the Nd2Fe14B film, and the magnet film may become separated. Non-Patent Document 4 additionally describes that it is possible to form a Nd—Fe—B film that is free from separation, even with a thickness of 2 μm, by forming a MoSi2 strain buffer film having a thickness of 50 nm on a Si substrate as a means for alleviating the stress in the heat treatment. Nevertheless, the film thickness of 2 μm is insufficient for extracting a sufficient magnetic field from the film surface to the outside due to the demagnetizing field in the film, and a film having a thickness of at least 10 μm or more is demanded. Meanwhile, when there is a difference in thermal expansion rate between the substrate and the film, the stress applied on the film will increase as the film thickness is increased, and film separation tends to occur more easily. Thus, a strain buffer film material that is free from the generation of separation even when depositing a thick Nd—Fe—B film on a Si substrate has been awaited for many years.
The present inventors previously developed a technique which enables the stable deposition of a Nd—Fe—B film having a thickness of 10 μm to 1.2 mm on a Ta substrate via laser deposition using a pulsed YAG laser. This deposition method is characterized in that there is superior compositional transcription between the target and the film, and the deposition rate is faster than the sputtering method by one order of magnitude or more. Furthermore, Non-Patent Document 5 describes that it is possible to deposit a Nd—Fe—B film, which is free from separation up to a maximum film thickness of 20 μm, on a Si substrate via pulsed laser deposition by interposing a Ta film having an intermediate value of the thermal expansion coefficient of Si and the thermal expansion coefficient of Nd2Fe14B. Nevertheless, when a film having a thickness exceeding 20 μm is formed, there are problems in that separation occurs between the Nd—Fe—B film and the Ta film and a fracture is generated inside the Si substrate.